What is the error message exactly?

Unable to write the email to the mailbox. Make sure the file system allows you write privileges and you have enough disk space to copy the mailbox.

C:/ 612 gb free, D:/ 424gb free

That wording leaves me more than suspicious that the issue is server side, not Thunderbird side of the transactions.

Select the inbox for the account and right click it. Select the properties entry. In the properties dialog select the quota tab.

What does it say quota usage is??

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.